Geochemical evolution of high arsenic groundwater in a typical area of the Hetao Basin, Inner Mongolia

Based on monitoring of groundwater levels and chemical conponents at different aquifers in a typical hydrogeological section of the Hetao basin, spatial and temporal variations in As, redox-sensitive parameters (including Eh, total dissolved Fe, Fe(Ⅱ)) were assessed, and the formation mechanisms of high As groundwater were evaluated. Results showed that groundwater levels were affected by agricultural irrigation in both groundwater irrigation area and surface water irrigation area. Spatial and temporal variations of groundwater As were observed. Spatially, groundwater As concentrations increased with the increasing sampling depths. In aquifers at depths <10 m, groundwater As was less than 5.0 μg/L, between 18.2 and 217 μg/L at depths 10~15 m, and between 38.3 and 226 μg/L at depths 15~25 m. Affected by agricultural irrigation, groundwater As generally increased with the increase in groundwater levels. The reason for the increases in As concentrations with increasing water levels is that the fluctuation of groundwater level induced the change of groundwater redox conditions. The major hydrogeochemical processes controlling groundwater As concentrations included reductive dissolution of Fe(Ⅲ) oxides and bacterial sulfate reduction.